1. Field of the Invention
This invention is in the field of gamma ray inspection devices for tubular products and the like employing an improved calibrating block which prevents the sensing system from being overloaded when no tubular product is present, and also provides the operator with a means for visually detecting the presence of wall thicknesses which are less than a required minimum.
2. Description of the Prior Art
It is common practice to check reconditioned pipe used in oil wells to make sure that the wall thickness of the pipe is still above a predetermined minimum. The wall thickness can be reduced, for example, by rubbing, by the rods used in the oil well, or in some cases by corrosion. Inspection of such pipes is necessary if catastrophic failure is to be avoided.
Testing services are now being provided whereby gamma ray scanning equipment is sent to the site and the scanning is performed on the job. A conveyor system is set up which delivers individual lengths of tubing or pipe into a scanning head which may be supported, for example, on the back of a truck. The conveyor moves the tubular product axially through the scanning head, and the head is rotated at a velocity of about 120 rpm so that a helical scan is produced. The amount of gamma ray attenuation in passing through the pipe or the tubular product is detected by a suitable detector device such as a scintillation counter, and the electrical impulses from the counter are passed to a recording device such as a pen-type recorder and/or a thickness indicator gauge so that the operator can continuously monitor the tubular products being tested.
Since the feed of pipes to be inspected is intermittent, there is usually a significant gap between pipes being tested. As a result, after one pipe has been passed through the rotating head, the gamma ray beam strikes the scintillation crystal detector and overloads the electronic sensing system. When this happens, it takes several minutes for the sensing system to be restored to operative condition.
The problem of proper calibration is also important if the sensing system is to give reliable results. The American Petroleum Institute sets up standards which classify pipe by the reduction of wall thickness. For example, if the wall thickness is reduced by a factor of in excess of 121/2% of the original nominal wall thickness, the pipe must be reclassified with a lower classification. It therefore becomes important for the operator to be able to sense when this amount of wall thickness reduction has occurred.
In the past, it has been common practice to attempt calibration by laying a hacksaw blade on the pipe and passing it through the rotating head. Since the thickness of the hacksaw blade is known, some measure of calibration can be achieved by using appropriate multipliers to determine a spot on the recording chart which would indicate the nominal wall thickness for the particular pipe being tested. However, this method is not particularly satisfactory since it requires additional computation which computation must be repeated every time a different diameter pipe is sent through the scanning device.